Origin of Visible Light Photoactivity of the CeO₂/ZnO Heterojunction

The synthesis of a mixed CeO2–ZnO oxide results in a photocatalyst active under visible light. The characterization of the new material shows that Ce does not enter as a dopant in ZnO but rather forms isolated CeO2 nanoparticles supported on the surface of larger particles of the more abundant zinc oxide phase. The as obtained material exhibits a band gap corresponding to UV light (∼3.3 eV), but nevertheless. it shows a relevant photoactivity under irradiation with photons with λ > 420 nm (visible light). The working hypothesis is that visible light irradiation leads to a charge separation and stabilization of a fraction of the carriers connected with the formation of the CeO2/ZnO interface. This phenomenon has been investigated by means of several methods. A specific EPR-based approach allowed to monitor and quantify the charge separation following the formation of holes in the valence band (VB) of the two materials. More complex is detecting the nature of the excited electrons, as this involves the formation of EPR invisible Ce3+ ions by trapping the electrons into localized 4f states of Ce ions at the interface between the two oxides. DFT calculations provide a rational for some of the observed phenomena and a basis for the discussion of the band alignment of the two systems as a consequence of the formation of a heterojunction. The theoretical results show that indeed electrons can be excited at the interface from the VB of the two oxides into the Ce 4f states with photons of 2.3 eV, thus justifying the occurrence of a visible-light activity despite the higher band gap of the two materials.